To really understand this class,
you will need to start with the Class::MOP::Attribute documentation.
This class can be understood as a set of additional features on top of the basic feature provided by that parent class.

This provides a shorthand for specifying the reader,
writer,
or accessor names.
If the attribute is read-only ('ro') then it will have a reader method with the same attribute as the name.

If it is read-write ('rw') then it will have an accessor method with the same name.
If you provide an explicit writer for a read-write attribute,
then you will have a reader with the same name as the attribute,
and a writer with the name you provided.

Use 'bare' when you are deliberately not installing any methods (accessor,
reader,
etc.) associated with this attribute; otherwise,
Moose will issue a deprecation warning when this attribute is added to a metaclass.

isa => $type

This option accepts a type.
The type can be a string,
which should be a type name.
If the type name is unknown,
it is assumed to be a class name.

Attribute (x) does not pass the type constraint (Int) with 'forty-two'

Before setting the value, a check is made on the type constraint of the attribute, if it has one, to see if the value passes it. If the value fails to pass, the set operation dies with a "throw_error".

Any coercion to convert values is done before checking the type constraint.

If, after installing all methods, the attribute object has no associated methods, it throws an error unless is => 'bare' was passed to the attribute constructor. (Trying to add an attribute that has no associated methods is almost always an error.)